1. Field of the Invention
The present invention refers to a multifunctional inking station for a flexographic printing machine, more particularly for a flexographic printing machine adapted to print web-like or plate-like workpieces such as webs or sheets of paper, cardboard or corrugated cardboard.
2. Description of the Background Art
Flexographic printing machines that are known up to now generally comprise a printing station including a plate cylinder to which the ink is transferred by means of a textured cylinder, which is better known in the art as an "anilox cylinder". The surface of this textured cylinder includes a plurality of cells, the density and distribution of which depend on the kind of printing work to be performed. This textured cylinder can be supplied with ink in more ways than one. A first way to supply this textured cylinder with ink is to contact it with a pad roller. The ink is supplied by means of a pump in an area located above the contact point between the textured cylinder and the pad roller. The ink is thereby layered between these two devices so as to wipe the textured cylinder, leaving ink only in its cells. This solution is suitable for most of the current printing work not requiring a high-grade quality. However, this arrangement allows medium quality printing of large plain surfaces called "tint blocks" by workers in the art.
Instead of supplying ink in the area above the contact point between the textured cylinder and the pad roller an alternate arrangement makes use of a transfer roller soaking in an ink fountain. In this case, the transfer roller deposits the ink on the pad roller at a point located before the contact point between the textured cylinder and the pad roller. Such an arrangement is, among others, described in the patent U.S. Pat. No. 4,526,102.
Another way to supply ink to the textured cylinder is to contact it with a chambered doctor blade. This chambered doctor blade consists of a chambered doctor blade body in the form of a U-shaped hollow bar whose part facing the textured cylinder comprises two doctor blades and whose two ends are sealed by two sealed joints. In this case, the ink is introduced into the chambered doctor blade body by means of a pump. The doctor blade is placed at a circumferential position in the direction of rotation of the textured cylinder, between the textured cylinder and its contact point with the printing plate cylinder. This will act to limit the amount of ink film deposited on the textured cylinder so that the ink only remains in the cells of the textured cylinder. The other doctor blade constitutes a prescraping and sealing means for the chambered doctor blade.
The utilization of a chambered doctor blade incorporating a metallic blade is particularly suited to produce work for which a high-grade printing quality is required. Effectively, due to the stiffness qualities of the doctor blades, it is easy to control very exactly the thickness of the ink film to be deposited on the textured cylinder. On the other hand, this method has the drawback of not being able to ensure a good quality for the printing of large surfaces ("tint blocks") since it does not allow supply of enough ink to the textured cylinder. Also, for the printing of large "tint blocks" it would be necessary to replace the textured cylinder by another cylinder a greater number of cells.
The wiping of the ink film on the textured cylinder depends, among other factors, on the material employed for the doctor blades, on their thickness, and on the angle at which the doctor blades are placed at their contact point with the textured cylinder. In order to improve the printing of large surfaces ("tint blocks"), workers in the art have tested and finally chosen thicker doctor blades, made of a plastic material as it is described in the patent U.S. Pat. No. 5,027,513. This solution, although better than the pad roller, has substantially the same drawbacks when requiring a high-grade printing quality for fine works which only include small "tint blocks" without changing the textured cylinder.
In order to improve the printing quality in accordance with the kind of work to be produced, the combined use of a pad roller inking station and a doctor blade inking station has been suggested. Such a combination is described in the patent EP 0 382 347 B1. The result obtained depends, however, on the drawbacks arising from the use of the inking station including a pad roller. The solution resulting from the combination has certainly allowed improve printing quality by enabling easy change from one printing mode to the other printing mode, in accordance with the work to be performed, but presents not inconsiderable drawbacks with respect to the cost of producing the station. Another drawback that should be noted lies in the fact that it is not possible to use substantially identical devices for making the inking station and that in most cases it would be necessary to change the textured cylinder, which is a difficult dismounting operation.
The object of the present invention is aimed at eliminating entirely or partly, the abovementioned drawbacks.